Thunderbird places messages into the sent items folder, but they are not received. The error "too many concurrent SMTP connections" is shown intermittently
the mail server sent an incorrect greeting, too many concurrent SMTP connections. When this happens, on an irregular basis, the message goes into the Sent Items folder, and has apparently been sent, but the recipient, when asked later, has not received it.
It also sometimes screws up the date order in the sent items folder, without the folder being clicked on, and puts two or three very old emails at the top as if they have just been sent.
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'Too many connections' means what it says: too many devices are trying to send messages via your account at the same time. All mail servers have limits to the traffic they can handle, and error messages like this are produced when those limits are exceeded. The message comes from the mail server, not from Thunderbird, so you need to talk to your email provider about it. They should publish the limits somewhere on their website. They may also have a helpline you can contact for an explanation. It's worth checking with them about the undelivered message too. If Thunderbird records it as sent then Thunderbird has probably received confirmation that it's been received by the server. Look at your mail account via the website and see if it's in the sent folder there. If it is, you can ask your mail provider to find out if it was transmitted to the recipient's account.
I find it very interesting that you say this, because numerous 'bug report' messages in Mozilla have this too many connections problem and the ISP is NOT to blame.
We also only appear to have started having the problem when we changed over to Thunderbird from Windows Live Mail.
See https://bugzilla.mozilla.org/show_bug.cgi?id=813537 as an example, as well as the other bug reports it refers to.
You could also look at https://bugzilla.mozilla.org/show_bug.cgi?id=797814
I see you've now gone rather quiet.
I need to get this resolved, as running a business with an unreliable email setup is just not on. I have waited for hours for a reply from a customer regarding his job, only to find out when I contact him again that he has never received the message.
I have the ISP looking into this problem as well, and if it turns out to be them I will hold my hand up and say so, but everything so far is pointing to Thunderbird.